Cellulose Composite Paper

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Computational Microscopy of PEDOT:PSS/Cellulose Composite Paper Aleksandar Y. Mehandzhiyski and Igor Zozoulenko* Laboratory of Organic Electronics, ITN, Linköping University, 60174 Norrköping, Sweden

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 46.161.60.242 on 05/06/19. For personal use only.

S Supporting Information *

ABSTRACT: Organic composite cellulose/conducting polymer materials have recently demonstrated their potential for energy storage applications. Here, we report a computational morphological study of CNF-PEDOT paper, which is composed of cellulose nanofibers (CNFs), and conducting polymer poly(3,4-ethylenedioxythiophene) blended with poly(styrenesulfonate) (PEDOT:PSS). With use of coarsegrained MARTINI molecular dynamics simulations, we study the aggregation of PEDOT:PSS on the cellulose surface, exploring different initial conditions for the self-assembly. The orientation of PEDOT chains with respect to the cellulose surface was studied, and the diffraction patterns were simulated. Finally, available experimental data were discussed and compared with calculated morphological models. KEYWORDS: cellulose, PEDOT, self-assembly, computational microscopy, coarse-grained, nanocomposites

1. INTRODUCTION Electrical energy production and consumption worldwide is constantly increasing, which drives the development of new approaches for the energy supply whenever it is necessary, more effective, and at lower cost. The performance of energy storage devices such as supercapacitors and batteries has been constantly improving in terms of power and energy densities. The increase of the performance can be achieved by tuning the properties of existing materials1 and devices or by the development of new materials with enhanced properties.2 Conducting polymers have gained increased popularity in recent years as promising materials for energy storage devices as well as in a variety of different applications, such as organic electronic ion pumps,3 electrochemical transistors,4 and drug delivery devices,5 etc. An important advantage of conducting polymers is that they exhibit both high electronic and ion conductivity simultaneously.6 In the same time, they do not possess any toxic elements and are easily processable and devices made with them can be easily scaled up. An example of such a widely used conducting polymer is poly(3,4-ethylenedioxythiophene) (PEDOT), which is typically blended with a negatively charged polyanion, such as poly(styrenesulfonate) (PSS) or molecular anion such as tosylate. PEDOT:PSS has been successfully used in devices such as sensors,7 ionic pumps, electrodes for supercapacitors, and solar cells8 to name a few, and such devices have broad applicability ranging from medical to energy storage applications.9 Another material gaining renewed interest recently is cellulose, including nanocellulose and cellulose nanocrystals. Moreover, cellulose is the most abundant biopolymer available © XXXX American Chemical Society

in nature, which is also easy to produce, process, and recycle. Advanced functional materials utilizing the unique nature of cellulose, such as transparent wood10,11 or conducting paper,12 have been a focus of recent research.13 Cellulose is also often used as a structural scaffold subjected to advanced functionalization such as chemical surface modification of fibrils.14 Malti et al.12 have combined the functionalities of PEDOT:PSS and cellulose nanofibers (CNFs) and successfully fabricated a mixed organic ion−electron conductor composite based on cellulose and conducting polymer (CNF-PEDOT paper). The CNF-PEDOT paper can be utilized for various applications such as fuel cells, organic transistors, sensors, ion pumps, supercapacitors, and batteries. The CNF-PEDOT composite paper is shown to exhibit enhanced electronic and ionic conductivity as compared to PEDOT:PSS.15 This enhancement was attributed to the effect of cellulose, which acts as a template for additional self-assembly of PEDOT oligomers. The authors speculated that PEDOT forms homogeneous film on the cellulose surface, thus enhancing the electron mobility. Recently, Belaineh et al.16 have observed by AFM microscopy the formation of PEDOT:PSS spherical aggregates on the cellulose surface. Other recent studies also report PEDOT:PSS/cellulose composite papers prepared from wood pulp nanocrystals17 and bacterial cellulose.18 Even though the reported studies provide an important experimental insight into the complex morphology of this material, the Received: February 13, 2019 Accepted: March 15, 2019

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DOI: 10.1021/acsaem.9b00307 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. Chemical structures and MARTINI coarse-grained models for cellulose,27 poly(3,4-ethylenedioxythiophene) (PEDOT),21 and polystyrenesulfonate (PSS)29 used in the present work. The colors of the atoms in the all-atom representation correspond to the coarse-grained bead to which they belong, shown in the bottom of each panel. The respective bead color and name, according to the MARTINI CG models used, are also presented in the legend of the figure.

2. METHODS AND MODELS

theoretical understanding of the morphology of the CNFPEDOT paper by means of computational techniques is still missing, and thus many aspects of its morphology still remain poorly understood. Molecular dynamics (MD) simulations have become a standard tool in many fields of material research, providing valuable insight into materials morphology at the atomic level. All-atomistic MD simulations can be computationally very demanding when addressing the length and time scales comparable to the experimental ones. However, coarse-grained MD methods and force fields, as compared with all-atom models, make it possible to study larger systems at longer time scales and to obtain morphology of materials directly comparable with experiments.19−21 For polymeric systems the coarse-grained approaches provided a valuable insight of the processes during the evaporation of solvent,20 the impact of the different solvents on the final morphology of bulk heterojunction organic solar cells,19 and ion diffusion in conducting polymers.21 In the present work, we apply computational microscopy to study the morphology of the CNF-PEDOT paper. Computational microscopy refers to the usage of computational tools, mostly MD simulations, to understand the complex dynamics and morphology of biosystems22−24 and thin films.25 In particular, with use of coarse-grained MD simulations, we study the self-assembly of PEDOT:PSS on a cellulose surface. To shed light on the structure of this otherwise morphologically complex material, we explore different initial conditions for the self-assembly and construct different initial simulation boxes containing cellulose fibril, PEDOT, and PSS−/PSSH (PSSH stands for the protonated PSS−). After that, a solvent evaporation scheme is applied where the water content is decreased from 80 to 10 wt % and the final morphology is obtained. Finally, 2D density maps, orientation angles of PEDOT with respect to cellulose surface, and diffraction patterns are calculated for the different morphologies. Comparison with available experimental data is done where appropriate.

2.1. Coarse-Grained Models. The MARTINI force field26 is used to model all molecular species in this work. A schematic representation of different species is shown in Figure 1, where different atoms of the same color correspond to the same coarsegrained (CG) bead which they belong to. Each glucose unit of cellulose is represented by three CG beads, following the model of Wohlert and Berglund.27 The cellulose nanofibril used in our study is comprised of 36 glucose chains, with each chain containing 100 glucose units. The total length of the fibril is 55 nm. The chains are arranged in a hexagonal shape with the Iβ configuration, containing two types of surfaces, hydrophilic (110) and hydrophobic (200). To model PEDOT, the recently developed MARTINI model by Modarresi et al.21 was used. In this model, each EDOT monomer is represented by five CG beads from which one is a virtual site located in the middle of the thiophene ring. The virtual site is charged (+0.333e), which corresponds to the oxidation level Cox = 33.3% of a pristine (i.e., as polymerized) PEDOT with one charge per three monomer units.28 The length of each PEDOT chain is 12 monomer units,25 and therefore the toal charge of each PEDOT oligomer is q = +4. PSS is modeled according to the MARTINI model developed by Vögele et al.29 This model uses three CG beads for the benzene ring, one bead for the backbone, and one bead for the sulfonate group. In general, the model has been developed for charged PSS, and therefore the natural choice for the sulfonate bead is the Qa MARTINI type. The protonated PSSH group, because of its highly polar character, is modeled by the P5 MARTINI bead according to Modarresi and Franco-Gonzalez.30 The chain length of PSS/PSSH has been kept to 50 monomer units for all the systems studied here. The total charge of the PSS− chain is q = −50. 2.2. Formulation of the Systems. The morphology of PEDOT:PSS itself is very complicated and challenging to simulate as the material is composed of three different species, PEDOT, PSS−, and PSSH (not counting Na+ and Cl− ions that are often present in the system). The addition of more degrees of freedom, as for example from cellulose, only makes it even more challenging. As mentioned in the Introduction, the detailed structure and morphology of CNFPEDOT paper are not known. Thus, in order to effectively explore the self-assembly of the polymers on the cellulose surface, we have designed different initial simulation boxes with random or semirandom initial distributions of the constituent species. Moreover, when cellulose and PEDOT:PSS are mixed and/or ultrasonicated in the experiments,16,17 it is not known what happens during this initial phase. It is possible that the PEDOT:PSS grains are affected (broken B


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Figure 2. Initial box configuration and final morphology obtained after 1.4 μs for (a) system 1, (b) system 2, (c) system 3, (d) system 4, and (e) system 5. The water molecules are removed for clarity. The size of the initial box is 22 × 22 × 65 nm3, and the final box is 6.5 × 6.5 × 65 nm3. PEDOT and PSS/PSSH chains consist respectively of 12 and 50 monomer units. The cellulose nanofibril is comprised of 36 glucose chains, with each chain containing 100 glucose units. The oxidation level of PEDOT is Cox = 33.3%. tibon et al.31 showed with use of X-ray photoelectron spectroscopy (XPS) that the PEDOT:PSS ratio near the cellulose surface is around 1:1.6, rather than 1:2.5, which is the bulk ratio. Since in our simulation setup we explore the region near the cellulose surface, we use a PEDOT:PSS ratio of 1:1.6 in all systems described above. This is a reasonable assumption, since the simulation box is not big enough to explore the bulk region of the CNF-PEDOT paper. In all systems the number of PEDOT and PSS−/PSSH chains was 200 and 56, respectively. After the computational boxes are filled with cellulose, polymer chains, and counterions (Na+), they are all solvated with polarizable water.32 Detailed information on the number of all species in each system is summarized in Table 1. System 1: Homogeneous distribution of PEDOT and homogeneous distribution of PSS−/Na+. The cellulose fibril is placed in the

down) due to the initial process of mixing/ultrasonication. Hence, this also motivates us to explore different initial conditions in our simulations. The simulated systems and the details of their preparation are described below and are also depicted in Figure 2. Each system contains one cellulose nanofibril comprised of 36 glucose chains, with each chain containing 100 glucose units. The cellulose nanofibril is placed in the middle of the computational box. PEDOT and PSS/PSSH chains consist respectively of 12 and 50 monomer units. The oxidation level of PEDOT is Cox = 33.3%. The size of the initial box is 22 × 22 × 65 nm3, and after the water evaporation its size is on average 6.5 × 6.5 × 65 nm3. We designed all of the systems described above to represent the experimental weight ratios of the constituents, which are 2:1 PEDOT/PSS:cellulose and 1:2.5 PEDOT:PSS.12,16 However, MonC


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ACS Applied Energy Materials Table 1. Number of PEDOT, PSS−, PSSH, Na +, and Water Molecules in Systems 1−5 system system system system system

1 2 3 4 5

PEDOT

PSS−

PSSH

Na+

water

200 200 200 200 200

56 28 16 16 50a

0 28 40 40 6

2000 600 0 0 0

95000 95000 95000 88085 88208

granular structure of PEDOT. Pre-equilibrated grains of PEDOT:PSS− are placed at different positions on the cellulose surface. The PSSH chains are randomly distributed between the grains, as depicted in Figure 2d. Each grain consists of 50 PEDOT chains and 4 PSS− chains. System 5: Mixture of PSS−/PSSH groups in a single chain. Generally, in one chain of poly(styrenesulfonate), both protonated and deprotonated sulfonate groups can exist. Because the detailed structure and composition of PEDOT-rich and PSS/PSSH-rich regions are not well understood, in this system we use PSS chains which have both PSS− and PSSH groups. Here, every third sulfonate groups is deprotonated, hence negatively charged, and therefore the charge of a PSS−/PSSH chain is q = −16. This matches PEDOT chains where every third monomer is positively charged. To every PSS chain, 4 PEDOT molecules were added and equilibrated; then the formed complexes were introduced to the simulation box containing the cellulose nanofibril. Additionally, PSSHs are also added to the box, in order to keep the PEDOT to PSS ratio the same as that for the other systems. In addition to these systems, we also prepared systems 1s−3s with a predefined seed of 20 PEDOT chains clustered together. In this way we explore if PEDOT seed can act as an initiator for further PEDOT aggregation. More detailed information on the preparation of systems 1s−3s can be found in the Supporting Information (SI). Note that experimental preparation of CNF-PEDOT usually also includes solvents such as DMSO as a second dopant and glycerol or poly(ethylene glycol) (PEG) as morphology enhancers.12,16 In our simulations, we do not include solvents and focus solely on the selfaggregation of PEDOT and PSS−/PSSH on the cellulose nanofibril, since the complexity of the system is high enough. The effect of solvents will be a subject of a future study. 2.3. Computational Details. The CG molecular dynamics simulations were carried out with the GROMACS-v5 simulation

This system is created with PSS− and PSSH groups in a single chain.

a

middle of the simulation box, while PEDOT, PSS−, and Na+ are placed randomly around the fibril. The chains of PSS− are negatively charged. Therefore, a corresponding number of Na+ counterions are added to keep the system electroneutral. System 2: Homogeneous distribution of PEDOT and homogeneous mixture of PSS/PSSH/Na+. This system is prepared in the same manner as system 1, with the difference being that half of the PSS− chains are replaced by PSSH. The number of Na+ is reduced as compared to that in system 1 to keep the electoneutrality of the system. System 3: PSS−/PSSH-rich regions and homogeneous distribution of PEDOT. One of the well-established features of PEDOT:PSS is its granular structure with PEDOT-rich and PSS/PSSH-rich regions.9,33−35 In order to account for the formation of the granular structure, the PSS− chains are predominantly placed in one part of the box, while the rest of the box is filled with PSSH. In the present system, PEDOT is distributed homogeneously. A number of PSS− chains are chosen such to compensate positively charged PEDOT, and therefore no Na+ ions were added. System 4: Predefined PEDOS:PSS− grains and homogeneous distribution of PSSH. This system is also designed to account for the

Figure 3. 2D density maps of (a) system 1, (b) system 2, (c) system 3, (d) system 4, and (e) system 5, where the upper plots show the total density in the simulation box and the bottom plot only cellulose and PEDOT densities. In panel f, the density maps from systems 3 and 4 are overlaid over the experimental AFM image of the CNF-PEDOT paper taken from one of the samples studied in ref 16, where two regions have been enlarged for clarity. The AFM image in panel f is courtesy of the authors of ref 16 (Belaineh et al.). The two images have been put to scale. D


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ACS Applied Energy Materials package.36 After the initial solvation of the simulation box, a short NVT (constant number of particles, volume, and temperature) and NPT (constant number of particles, pressure, and temperature) equilibrations of the solvent molecules were performed, while applying position restraints to the PEDOT, PSS, and cellulose molecules.21 After the water is equilibrated, a production run of 200 ns is carried out in the NPT ensemble, employing the Berendsen barostat at 1 bar.37 We perform water evaporation by removing solvent molecules in six steps with respective hydration levels Hy = 80, 70, 60, 50, 30, and 10 wt % of the total box weight, where at every evaporation step the system is run for 200 ns. After each water evaporation step we confirm that an equilibrium in the system is reached by examining that the total energy of the system has reached a plateau (SI Figure S1). After obtaining the morphology with Hy = 10 wt %, the system is run for an additional 200 ns with the Parrinello−Rahman barostat38 to obtain the final morphology. Periodic boundary conditions (PBCs) are applied in all spatial directions, where the initial box dimensions are set to 22 × 22 × 65 nm in the xyz directions, respectively. Semi-isotropic pressure of 1 bar is applied separately in xy and z directions, where only the xy dimensions are rescaled while the z dimension is kept fixed. This was done to prevent the interaction of the cellulose fibril with itself, since the fibril is oriented along the z direction, which is larger than the fibril length. The initial box size is chosen to be large enough to prevent finite-size effects on the self-assembly at the cellulose surface. The total simulation time is 1.4 μs. The temperature during the water evaporation was T = 343 K, and for the last 200 ns the temperature was decreased to 300 K, in order to mimic the experiments. The particle mesh Ewald (PME)39 method is used for the long-range electrostatic interactions with the cutoff of 1.2 nm. In the present study we calculate two-dimensional number-density maps as implemented in the GROMACS-v5 simulation package.36 Additionally, we also compute the orientation angles of the PEDOT thiophene rings with respect to the 110 and 200 cellulose surfaces, as well as the diffraction patterns20 for cellulose and PEDOT. The diffraction patterns are calculated by following the methodology presented by Alessandri et al.20 with the scattering intensity given by N

I(q) ∝ |F q|2 ∝

positively charged sodium ions, being significantly smaller in size than PEDOT, can compete with PEDOT and thus replace it in complexes with PSS−. The effect of Na+ is less pronounced in system 2, where some free cellulose surface can be noticed in Figure 3 b. 3.1.2. Systems 3 and 4. System 3 is prepared with the homogeneous initial distribution of PEDOT chains, but the initial box PSS− and PSSH are placed into two separate regions (to the left and to the right, respectively). During the water evaporation some of the PSS− chains diffused into the PSSHrich region. Nevertheless, it can be seen that these regions still remain predominantly occupied by respectively PSS− and PSSH chains; see Figure 2 c. PEDOT chains, initially randomly distributed, accumulate mainly in the PSS-rich region. PEDOT chains are also present in the PSSH-rich region. A visual inspection of the snapshot of the final morphologies (Figure 2c, right panels) shows that even in the PSSH-rich region PEDOT chains tend to aggregate with PSS− chains. System 4, presented in Figures 2 d and 3 d, is also designed with a predefined nonhomogeneous initial distribution. In system 3, PSS −/PSSH chains are initially distributed inhomogeneously, whereas system 4 is comprised of predefined PEDOT:PSS− aggregates. The simulations show that the structures of these aggregates do not change significantly from the predefined initial configuration. The aggregates do not coalesce with each other and remain mostly separated. In the snapshots of Figure 2 d two of the PEDOT:PSS grains are connected through PEDOT chains, while the remaining two aggregates remain separated. It should be stressed that while the initial distributions of the components are different for systems 3 and 4, the final morphologies for these systems are rather similar. Indeed, both of the systems are composed of PEDOT:PSS− aggregates (representing PEDOT-rich regions) and PSSH aggregates containing relatively few PEDOT and PSS− chains. The reason for this is a strong Coulomb interaction between positively charged PEDOT chains and negatively charged PSS− chains, which leads to a formation of the aggregates for the case of the inhomogeneous initial distribution of the components. 3.1.3. System 5. System 5 consists of homogeneously distributed predefined PEDOT:PSS−/PSSH complexes, where all PSS chains have both PSS− and PSSH groups on every chain; see Figures 2 e and 3 e. The strong electrostatic interaction between PEDOT and PSS chains prevents a phase separation between them; thus, the PEDOT:PSS−/PSSH complexes remain very stable. The complexes remain homogeneously distributed in the system during the entire simulation run until the equilibrium is reached. As a result, PEDOT chains are also homogeneously distributed in the system. 3.1.4. Systems 1s−3s. In these systems we introduced predefined PEDOT seeds clustered together. The calculations show that systems 2s and 3s result in the aggregated morphology, whereas in system 1s the initial PEDOT seed is homogeneously dispersed; see the Supporting Information. On the basis of all calculated morphologies (systems 1−5 and systems 1s−3s), we thus conclude that if an inhomogeneous initial distribution, whether a positively or negatively charged component, is introduced (i.e., PSS− in system 3 or PEDOT in systems 2s and 3s, or PEDOT:PSS aggregates in system 4), we arrive to the final aggregated structure. System 1s (with the highest concentration of homogeneously distributed Na+ ions)

2

∑ Zj exp(iq·rj) j=1

(1)

where q is the reciprocal space vector, rj is the position vector of atom j, and Zj is the atomic number of atom j. The orientation angles and diffraction patterns are calculated with the MDAnalysis package.40,41 The VMD package42 is used to prepare all of the simulation snapshots.

3. RESULTS AND DISCUSSION 3.1. Morphology of CNF-PEDOT Composites. This section focuses on the mophological description of the studied systems 1−5. Panels a−e of Figure 2 show snapshots for the initial (80 wt %) and final (10 wt %) box configurations, as well as PEDOT and PSS−/PSSH distributions in the final box configurations for all of the systems simulated. In Figure 3, the 2D number-density maps of the final morphology of the systems are depicted and compared to the experimental AFM images of the CNF-PEDOT paper by Belaineh et al.16 The upper plots in Figure 3 a−e represent the total density of the system, including all of the molecular species, while the bottom plots depict only the cellulose and PEDOT densities. 3.1.1. Systems 1 and 2. The calculated morphologies for systems 1 and 2 are shown in Figures 2 a,b and 3 a,b. For these systems PEDOT is homogeneously distributed across the simulation box, covering almost entirely the available cellulose surface. This is most pronounced for system 1, where PSS− is also evenly distributed across the simulation box. System 1 is also the system with the highest content of Na+ ions. The E


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Figure 4. Orientation of PEDOT molecules at the cellulose surface. Illustrative snapshots showing (a) predominantly edge-on; (b) face-on, and (c) mixture of face-on and edge-on on two adjacent cellulose surfaces. A definition of the orientation angle (ϕ) of thiophene rings with respect to the cellulose surface is shown in panel a. Distributions of ϕ for different cellulose surfaces for (d, i) system 1, (e, j) system 2, (f, k) system 3, (g, l) system 4, and (h, m) system 5.

homogeneous distribution of PEDOT in the final morphologies not showing any bead-like structures. We therefore conclude that these systems do not describe the experimental CNF-PEDOT paper. On the contrary, the bead-type structure is clearly visible in the calculated density maps of systems 3 and 4. (We note once again that despite the difference in the initial morphologies of systems 3 and 4, the final morphology for these systems is rather similar). The density map for system 3 is overlaid onto the AFM image16 of the CNF-PEDOT paper and is presented in Figure 3 f. It can be seen that the simulated morphology closely resembles the AFM image, with bead-like PEDOT aggregates separated by regions of a lower density between them being clearly visible on both pictures. The material deposited onto or around the fibril contributes to the higher density on the density maps and the brighter regions in the AFM image. It is difficult to deduct from the experimental AFM pictures if between these regions the cellulose surface is

results in the homogeneous structure because of the effect of Na+ ions, which overweighs the effect of the PEDOT seed. 3.1.5. Comparison to Experiment. Calculated morphologies are now compared to the available experimental results. The AFM topography images of the CNF-PEDOT paper have been recently reported by Belaineh et al.16 The images exhibit the evidence of bead-like PEDOT structures aligned along the cellulose nanofibrils. The beads are of a typical size of 13 nm in diameter, and they form a continuous bead path through the structure, which presumably provides percolation paths for electrons resulting in a high electronic conductivity of this material. The calculated 2D density maps of the 5 systems considered in this study are shown in Figure 3. Systems with the initial homogeneous distribution of PEDOT (systems 1 and 2), as well as a system with the initial homogeneous distribution of PEDOT:PSS−/PSSH complexes (system 5), result in a F


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Figure 5. Computed scattering curves for (a) cellulose and (b) PEDOT, for all of the simulated systems 1−5.

angle between vectors ncell and nth, which are the normal vectors to the cellulose surface and to the thiophene rings, respectively, as illustrated in Figure 4 a. The orientation angle is calculated as a function of the distance of the thiophene ring from the cellulose surface. The character of the orientation of PEDOT chains with respect to a surface (face-on vs edge-on orientations) was recently discussed by Franco-Gonzalez et al.45 for different substrates. (The chain is oriented face-on if the angle between a thiophene ring and a cellulose surface is close to 0°, and edge-on, if it is closer to 90°.) We first note that a minimum distance between PEDOT chains and the cellulose surface is around 0.4 nm, which is comparable to the π−π stacking distance between PEDOT chains. Second, the character of the orientation of PEDOT chains is rather insensitive to the type of surface (hydrophobic vs hydrophilic) and primarily depends on the method of preparation of the system. For example, the orientations in systems 2 and 3 (Figure 4 e,f and Figure 4 j,k) are predominantly face-on with angles below 30° for both 110 and 200 surfaces. This is different for the rest of the systems where a mix of face-on and edge-on orientations coexist on both surfaces. For systems 4 and 5 the orientation of PEDOT chains is largely determined by the predefined PEDOT:PSS− and PEDOT:PSS−/PSSH complexes. In these predefined complexes the PEDOT chains have completely random orientations and therefore the distribution of ϕ is practically not affected by the presence of the cellulose fiber. For system 1 we speculate that the mix of the face-on and edge-on orientations is caused by the effect of Na+ ions that partially penetrate into the space between PEDOT and cellulose, and thus “screen” the effect of the cellulose surface on the PEDOT chain orientation. Despite their relatively short length, PEDOT chains possess certain flexibility,25 which allows for a variety of different chain orientations at the surface of cellulose. For example, Figure 4 c shows a mixture of face-on and edge-on orientations of two PEDOT chains adsorbed on two neighboring surfaces of the cellulose fibril. One PEDOT chain is in the face-on orientation on one cellulose surface, while another chain is in the edge-on configuration on the neighboring surface. However, π−π stacking between the two chains is still possible due to the chain flexibility. Since the most important factor for interchain electron transfer between PEDOT chains is the effective π−π stacking,46 that can be achieved even if there is a mixture of the face-on and edge-on orientations in one system. 3.3. Simulated Diffraction Curves. An advantage of the theoretical calculations is that the signals from the different components can be separated from the overall diffraction

covered with some PEDOT:PSS chains or is completely bare. However, from the simulations it is visible that most of the cellulose surface is uncovered in the region between the two aggregates, and the aggregates are interconnected by a network of PEDOT chains. Since the bulk of the CNF-PEDOT paper is conductive, the grains either need to overlap or to be connected by PEDOTs. The simulation results suggest that the latter is most likely to happen. In the experiments, cellulose is usually mixed with a water dispersion of PEDOT:PSS (which already contains PEDOT:PSS grains28) with a high shear mixer.16 Alternatively, it is mixed and afterward ultrasonicated.17 These seemingly similar experimental protocols yield however completely different morphologiesa bead-like structure of PEDOT:PSS on the cellulose surface16 for the former protocol and a uniform (homogeneous) film of PEDOT:PSS at the cellulose crystals17 for the latter one. In our simulations we observe both uniform (systems 1, 2, and 5) and non-uniform (systems 3 and 4) distributions of PEDOTs, depending on the initial preparation of the system. We argue that for the experimental protocol of ref 16, the PEDOT:PSS beads presented already in the dispersion are not affected considerably by mixing only (i.e., the PEDOT:PSS beads remain mostly intact and after mixing they are adsorbed on the cellulose surface). This is apparently modeled by the initial conditions in systems 3 and 4. On the other hand, the ultrasonication of the sample likely results in the separation of PEDOTs from the PSS matrix. The sound waves generated by the ultrasonication migrate through the sample and induce pressure variations and cavitations which grow and collapse and thus transform the sound waves into mechanical energy.43,44 Therefore, in ref 17. the PEDOT:PSS grains have likely been destroyed and the initial distribution of PEDOT and PSS is homogeneous, corresponding to our systems 1, 2, and 5, for which we also obtain uniform PEDOT:PSS film on the cellulose surface. Thus, the initial conditions created in the experiments by applying different protocols to a large extent determine the final morphology of the PEDOT/cellulose composite, similar to the simulations. The initial computational time in our simulations corresponds to the time in the experiments when the mixing/ ultrasonication is finished and the sample is left to dry. It should be also noted that Belaineh et al.16 achieved orders of magnitude higher conductivity than Alam et al.17 3.2. Orientation of PEDOT with Respect to the Cellulose Surface. The distribution of orientation angles (ϕ) of the thiophene rings with respect to the cellulose hydrophilic (110) and hydrophobic (200) surfaces are calculated and presented in Figure 4. ϕ is defined as the G


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morphologies closely resemble experimental AFM images with the bead-like structure. With this study we shed light on the otherwise complex morphology of CNF:PEDOT and established a framework for more in-depth computational investigations of this promising material for energy storage. Morphologies obtained by the coarse-grained simulations can serve as a starting point for multiscale calculations of the electron mobilities45,46 and ionic conductivity simulations.21

pattern, which is apparently not possible in the experimental data. The simulated diffraction patterns of systems 1−5 calculated separately for cellulose and PEDOT are shown in Figure 5 a,b. All of the diffraction patterns have the same overall shape, and only the magnitude of the peaks for different systems differ slightly. This means that the local microscopic arrangement into crystallites is rather similar for all studied systems, even though the global arrangement is different, as discussed in section 3.1. The diffraction pattern of PEDOT exhibits a well-defined π−π stacking peak at q ≈ 1.6 Å−1; see Figure 5. This value corresponds to the stacking distance of 0.39 nm, which is slightly larger than the stacking distance between PEDOT chains obtained from the atomistic MD simulations, as well as from the experimental measurements. The discrepancy in the stacking distance is a well-known feature of the MARTINI force field,20,21 which is due to the large radius of the CG beads. It should be stressed that this discrepancy does not affect the overall calculated morphology. The width of the calculated π−π stacking peak is similar to the one reported in previous studies of PEDOT and corresponds to 3−5 chains in crystallites.25,45 The characteristic cellulose peaks47 corresponding to 11̅0, 110, 200, and 004 surfaces are presented in Figure 5 The scattering patterns presented here are in agreement with the experimental GIWAXS16 measurements of the CNFPEDOT paper. In our simulations we did not observe any lamellar structures of PEDOT (corresponding to the 100 peak at q ≈ 0.25 Å−1), which is also in agreement with the experimental data,16 where the lamellar structures of PEDOT were only observed when PEDOT:PSS has been mixed with pulp, but not with the nanofibrillated cellulose.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00307. Three additional simulation setups and the corresponding results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Aleksandar Y. Mehandzhiyski: 0000-0001-5671-4545 Igor Zozoulenko: 0000-0002-6078-3006 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the authors of ref 16, in particular, Dr. Dagmawi Belaineh Yilma, for numerous discussions and for supplying the AFM image of CNF-PEDOT paper used in Figure 3. This work was supported by the Swedish Energy Agency (Grant 43561-1), the Swedish Research Council (Grant 2016-05990), the Peter Wallenberg Foundation (Grant PWS-2016-0010), and Åforsk (Grant 17-367). I.Z. thanks the Advanced Functional Material Center at Linköping University for support. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at NSC and HPC2N.

4. CONCLUSIONS Herein, we presented computational microscopy study of the morphology of the CNF:PEDOT composite paper utilizing coarse-grained MARTINI molecular dynamics simulation. In order to effectively explore self-assembly of PEDOT on the cellulose surface, we have designed 5 different initial systems with homogeneous and nonhomogeneous initial distributions of the constituent species (PEDOT:PSS, PSSH, CNF, ions, and water). A solvent evaporation scheme was then performed to obtain the final morphology of the systems at hand. Finally, 2D density maps, orientation angles of PEDOT with respect to cellulose surface, and diffraction patterns were calculated for the different systems. The calculated morphologies for the studied systems were compared with the recent experimental data of Belaineh et al.16 showing the evidence of bead-like PEDOT structures aligned along the cellulose nanofibrils. We find that the systems with the homogeneous initial distribution of the components (systems 1 and 2), and a system where all PSS chains have both PSS− and PSSH groups on every chain (system 5), do not lead to the formation of PEDOT aggregates on the cellulose fibers, and thus these systems do not describe the experimental CNF-PEDOT paper. On the contrary, systems 3 and 4 with predefined initial PSS-rich regions and PEDOT:PSS grains result in a granular final morphology similar to that with PEDOT-rich regions. If we introduce an inhomogeneous initial distribution of any component (i.e., PSS− in system 3 or PEDOT in systems 2s and 3s, or PEDOT:PSS− aggregates in system 4), we always arrive to the aggregated structure. The calculated 2D density maps for these

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